The identification and selection of a solid form of a pharmaceutical compound is complex, given that a change in a solid form may affect a variety of physical and chemical properties, which may provide benefits or drawbacks in processing, formulation, stability and bioavailability, among other important pharmaceutical characteristics. Potential pharmaceutical solids include crystalline solids and amorphous solids. Amorphous solids are characterized by a lack of long-range structural order, whereas crystalline solids are characterized by structural periodicity. The desired class of pharmaceutical solid depends upon the specific application; amorphous solids are sometimes selected on the basis of, e.g., an enhanced dissolution profile, while crystalline solids may be desirable for properties such as, e.g., physical or chemical stability (see, e.g., S. R. Vippagunta et al., Adv. Drug. Deliv. Rev., (2001) 48:3-26; L. Yu, Adv. Drug. Deliv. Rev., (2001) 48:27-42).
Whether crystalline or amorphous, potential solid forms of a pharmaceutical compound include single-component and multiple-component solids. Single-component solids consist essentially of the pharmaceutical compound in the absence of other compounds. Variety among single-component crystalline materials may potentially arise from the phenomenon of polymorphism, wherein multiple three-dimensional arrangements exist for a particular pharmaceutical compound (see, e.g., S. R. Byrn et al., Solid State Chemistry of Drugs, (1999) SSCI, West Lafayette). The importance of discovering polymorphs was underscored by the case of Ritonavir, an HIV protease inhibitor that was formulated as soft gelatin capsules. About two years after the product was launched, the unanticipated precipitation of a new, less soluble polymorph in the formulation necessitated the withdrawal of the product from the market until a more consistent formulation could be developed (see S. R. Chemburkar et al., Org. Process Res. Dev., (2000) 4:413-417).
Additional diversity among the potential solid forms of a pharmaceutical compound may arise from the possibility of multiple-component solids. Crystalline solids comprising two or more ionic species are termed salts (see, e.g., Handbook of Pharmaceutical Salts: Properties, Selection and Use, P. H. Stahl and C. G. Wermuth, Eds., (2002), Wiley, Weinheim). Additional types of multiple-component solids that may potentially offer other property improvements for a pharmaceutical compound or salt thereof include, e.g., hydrates, solvates, cocrystals and clathrates, among others (see, e.g., S. R. Byrn et al., Solid State Chemistry of Drugs, (1999) SSCI, West Lafayette). Moreover, multiple-component crystal forms may potentially be susceptible to polymorphism, wherein a given multiple-component composition may exist in more than one three-dimensional crystalline arrangement. The discovery of solid forms is of great importance in the development of a safe, effective, stable and marketable pharmaceutical compound.
Notably, it is not possible to predict a priori if crystalline forms of a compound even exist, let alone how to successfully prepare them (see, e.g., Braga and Grepioni, 2005, “Making crystals from crystals: a green route to crystal engineering and polymorphism,” Chem. Commun.: 3635-3645 (with respect to crystal engineering, if instructions are not very precise and/or if other external factors affect the process, the result can be unpredictable); Jones et al., 2006, Pharmaceutical Cocrystals: An Emerging Approach to Physical Property Enhancement,” MRS Bulletin 31:875-879 (At present it is not generally possible to computationally predict the number of observable polymorphs of even the simplest molecules); Price, 2004, “The computational prediction of pharmaceutical crystal structures and polymorphism,” Advanced Drug Deliver Reviews 56:301-319 (“Price”); and Bernstein, 2004, “Crystal Structure Prediction and Polymorphism,” ACA Transactions 39:14-23 (a great deal still needs to be learned and done before one can state with any degree of confidence the ability to predict a crystal structure, much less polymorphic forms)).
Cocrystals are crystalline molecular complexes of two or more non-volatile compounds bound together in a crystal lattice by non-ionic interactions. Pharmaceutical cocrystals are cocrystals of a therapeutic compound, e.g., an active pharmaceutical ingredient (API), and one or more non-volatile compound(s) (referred to herein as coformer). A coformer in a pharmaceutical cocrystal is typically selected from non-toxic pharmaceutically acceptable molecules, such as, for example, food additives, preservatives, pharmaceutical excipients, or other APIs. In recent years, pharmaceutical cocrystals have emerged as a possible alternative approach to enhance physicochemical properties of drug products. The variety of possible solid forms creates potential diversity in physical and chemical properties for a given pharmaceutical compound.
The compound chemically named 1-ethyl-7-(2-methyl-6-(1H-1,2,4-triazol-3-yl)pyridin-3-yl)-3,4-dihydropyrazino[2,3-b]pyrazin-2(1H)-one and tautomers thereof (collectively referred to herein as “Compound 1”) are disclosed in U.S. Pat. No. 8,110,578, issued on Feb. 7, 2012, and International Pub. No. WO 2010/062571, the entireties of each of which are incorporated by reference herein.
Citation or identification of any reference in Section 2 of this application is not to be construed as an admission that the reference is prior art to the present application.